Asterionella,
a colony of pennate diatoms photographed by Jan Parmentier.

What
are diatoms?

Diatoms
are delicate unicellular organisms that have a yellow-brown chloroplast that enables
them to photosynthesize. Their cell walls are made of silica almost like a glass
house. The construction of the cell wall, called the frustule, consists of two
valves that fit into each other like a little pill box.

The
colour of the chloroplast is yellow-brown instead of the green we know of other
creatures that use light as a source for energy.

There
are two different groups of diatoms, the pennates which are pen-shaped and the
centric which are like a cylinder. In fresh water most diatoms you will see are
of the pennate type.

Where
can you find diatoms?

At
the end of the winter they are most numerous in fresh water. They will cover surfaces
of aquatic plants or poles and wooden borders of ponds. If you like to study them
you can scrape the brown growth with a flat piece of plastic. You can also use
a sponge.

For the
free living (plankton) species a special fine mesh plankton net is very useful.

Yellow-brown
chloroplasts can be seen in this Cymatopleura.

Many
species of diaom stay connected after the cells divide. They form colonies, long
chains. Sometimes only the tips are connected and they form a zig zag pattern.

The
cells of diatoms are ideal subjects for study under the microscope. They show
complex patterns of very fine punctures and they often have all kinds of ornaments.

The best way
to make the beautiful structures of the frustule visible is to remove the content
from the cell. Hydrogen peroxide can be used for this. But it is also possible
to find empty frustules of dead diatoms.

The
extremely fine pores in the frustule of certain species of diatoms are used to
test the resolving power of a microscope's lens.

Diatom's
locomotion

Pennate
diatoms, which developed later in the earth's history, are able to locomote in
a slow gliding fashion in the direction of the length of the cell. The mechanism
for this is still not well understood but it seems that through the slit alongside
the cell (the raphe) tiny microfibrils protrude. With these they can move over
a surface. In the picture of the cleaned diatom above the raphe can be seen as
the thin central horizontal line.

There
are two different groups of diatoms, the pennates which are pen-shaped and the
centric which are like a cylinder. In fresh water most diatoms you will see are
of the pennate type.

In
marine waters the variety of body shapes is much greater. In the ocean they form
the main part of phytoplankton, the photosynthetic organisms that float with the
current. This image shows the top view of a large Coscinodiscus, a marine
species that can just be seen with the naked eye.

Clearly
visible is the fine net-like structure of the siliceous cell wall. The yellow-brown
chloroplasts used for photosynthesis are also easy to see.